Photodissociation of C₆H₅I, C₆F₅I, and related iodides in the ultraviolet

Abstract

The quantum yields of I^∗(²P_½) production from iodobenzene and pentafluoroiodobenzene at five different dissociation wavelengths of 222, 236, 266, 280, and ~305 nm are presented and compared with those obtained from nonaromatic cyclic iodides (i.e., cyclohexyl iodide and adamantyl iodide). The I(²P_3/2) and I^∗(²P_½) atoms generated in the photolysis of the above iodides were monitored using a two-photon laser-induced fluorescence technique. From the measured I^∗ quantum yields, two general observations are made for aryl iodides. They are that (i) the I^∗ yield is influenced by the σ^∗←n as well as π^∗←π transitions at all photolysis wavelengths within the A band and (ii) there is a clear indication of a fluorine substitution effect on the dynamics of I^∗ production. The contribution from the benzene type π^∗←π transition varies with excitation wavelength. Fluorine substitution in aryl iodides is found to increase the I^∗ quantum yield similar to what is reported in alkyl iodides. The effect of fluorine substitution is more pronounced at the red edge of the A-band excitation than at any other wavelengths. This is explained by invoking the presence of a charge-transfer band arising due to the transition of a 5pπ nonbonding iodine electron to the π^∗ molecular orbital near the red edge of the A band. This charge-transfer state is coupled more strongly to the ³Q₁ state of the σ^∗←n transition in pentafluoroiodobenzene than in iodobenzene. The dynamics of I^∗ formation is found to be unaltered by ring strain in cyclic iodides except at the blue wing excitation. At the blue wing, B-band transitions affect the dynamics of I^∗ production in cyclic iodides, leading to the formation of more I^∗ from adamantyl iodide.